Technique for mitigation of polarization mode dispersion in fiber optic transmission links

ABSTRACT

A technique for PMD mitigation in an optical communications system that utilizes a plurality of polarization rotators in an optical fiber to continuously rotate the polarization state of at least one optical signal that propagates through the fiber. The optical fiber is segregated into a plurality of sections and the polarization rotators are disposed between adjacent sections of the fiber. The polarization state of at least one optical signal received from the first of a pair of adjacent sections is continuously rotated by the polarization rotator prior to being transmitted to the second of the pair of adjacent sections. The rotated optical signals are collected at a receiver and corrected for errors using, for example, forward error correction.

FIELD OF THE INVENTION

The present invention relates generally to optical communications, andmore particularly, to a system and method for mitigating the effects ofPolarization Mode Dispersion (PMD) in optical transmission media.

BACKGROUND

Optical communications have revolutionized the telecommunicationsindustry in recent years. The fiber optic medium provides the ability toefficiently transmit high bit rate signals through a low-loss medium.The development of modern high bandwidth techniques, and wavelengthdivision multiplexing (WDM) to permit the simultaneous transmission ofmultiple high bandwidth channels on respective wavelengths, has enableda tremendous increase in communications capacity. The last decade hasbeen seen efforts to increase capacity by taking advantage of the fiberoptic medium to the maximum extent possible.

Signals transmitted through an optical medium can be affected by PMD,which is a form of signal distortion that can be caused by subtlephysical imperfections in the optical fiber. In principle, an opticalfiber with a circular core has rotational symmetry, so that there is nopreferred direction for the polarization of the light carrying theoptical signal. However, during fabrication, jacketing, cabling, andinstallation, perturbations in the fiber that will distort this symmetrycan occur, thereby causing the fiber to “look different” to variousoptical polarizations. One of the manifestations of this loss ofsymmetry is “birefringence,” or a difference in the index of refractionfor light that depends on the light's polarization. Light signals withdifferent polarization states will travel at different velocities. Inparticular, there will be two states of polarization (SOPs), referred toas the “eigenstates” of polarization corresponding to the asymmetricfiber. These eigenstates form a basis set in a vector space that spansthe possible SOPs, and light in these eigenstates travels at differentvelocities.

A birefringent optical fiber transporting a modulated optical signal cantemporally disperse the resulting optical frequencies of the signal. Forexample, an optical pulse, with a given optical polarization, can beformed to represent a “1” in a digital transmission system. If thesignal is communicated through a medium with uniform birefringence(i.e., remaining constant along the length of the fiber), the SOPs canbe de-composed into corresponding eigenstates, thereby forming twoindependent pulses, each traveling at its own particular velocity. Thetwo pulses, each a replica of the original pulse, will thus arrive atdifferent times at the end of the birefringent fiber. This can lead todistortions in the received signal at the end terminal of the system. Inthis simple illustrative case, the temporal displacement of the tworeplicas, traveling in the “fast” and “slow” SOPs, grows linearly withdistance.

In a typical optical communications system, birefringence is notconstant but varies randomly over the length of the transmission medium.Thus, the birefringence, and therefore, the eigenstate, changes withposition as the light propagates through the length of the fiber. Inaddition to intrinsic changes in birefringence resulting fromimperfections in the fabrication processes, environmental effects suchas, for example, temperature, pressure, vibration, bending, etc., canalso affect PMD. These effects can likewise vary along the length of thefiber and can cause additional changes to the birefringence. Thus, lightthat is in the “fast” SOP in one section of fiber might become be in the“slow” SOP at another section of the fiber. Instead of increasinglinearly with distance, the temporal separations in the pulse replicaseventually take on the characteristics of a random walk, and grow withthe square root of the distance. Despite the local variations in thefast and slow states, it is understood that when the fiber as a whole isconsidered, another set of states can be defined that characterize thePMD for the entire fiber and split the propagation of the signal intofast and slow components. These “principal states” can be imaged (in amathematical sense) back to the input face, and used as an alternativebasis set. Thus, an arbitrary launch SOP will have components in each ofthe principal states, and distortion will result from the replication ofthe pulses after resolution into principal states and their differentialarrival times. While the physical process is described in the foregoingin a “global” as opposed to “local” sense, the basic impairment is thesame; distortion results from the time delay introduced in the pulsereplicas.

The above discussion relates to “narrowband” signals, i.e., having anarrow enough bandwidth that the optical properties of the fiber can becharacterized as operating at a single wavelength. This is commonlyreferred to as “first order PMD.” Birefringence, however, can also varywith wavelength, such that each section of fiber may have slightlydifferent characteristics, both in the magnitude and direction of thebirefringence. As a consequence, after a long propagation through anoptical medium, light from two neighboring wavelengths initially havingthe same polarization may experience what looks like a fiber with twodifferent characteristics.

Theoretically, PMD can be represented by a Poincare sphere, or “Stokes'space” representation. In this representation, the equations of motionfor SOPs and PMD at a given optical frequency are given by:∂s/∂z=β×s  (1a)∂s/∂ω=τ×s  (1b)∂τ/∂z=∂β/∂ω+β×τ  (1c)In these equations (which are in the “representation” space, not “real”space) “β” represents the birefringence of the fiber at position z, “s”represents the SOP of the light at position z, and “τ” represents thePMD. Generally, Eqn. (1a) states that birefringence causes therepresentation of the SOP to rotate about the “β” axis as lightpropagates through the fiber. Eqn. (1b) states that, when viewed at agiven position (e.g., the fiber output), the system's PMD causes the SOPto rotate about it as a function of optical frequency. In this regard,light launched at a given optical frequency will evolve to an SOP at theoutput, and if the optical frequency is then changed (but the launchpolarization remains the same), the SOP at the output will also begin torotate about the PMD vector, τ. Eqn (1c) states that the vectorcharacterizing PMD changes along the length of the fiber. The drivingterm in Eqn (1c), β′=∂β/∂ω, which we refer to as the “specific PMD,”describes the relationship of birefringence to optical frequency. Evenfor the simplest cases, there is usually a non-zero driving term (andthus PMD) for birefringent fibers. Based on the above, the vector s willsuffer infinitesimal rotations about the axis defined by β, and that therotation axis will change as β changes with distance (and parametricallywith time). However, the total evolution of s can be represented by asingle, finite rotation based upon Euler's theorem. If the signalbandwidth is large enough to experience these variations, it is commonlyreferred to as “higher order” PMD. Higher order PMD also leads to pulsedistortion as the optical bandwidth of the signal increases. As thebandwidth increases, the input signal can be decomposed into Fouriercomponents, with each propagated in accordance with the equationsdiscussed above, and the components collected at the output.

In the narrowband context, for illustrative purposes, the “concatenationrule” represented by the above equations states that the PMD of a givensection of fiber can be “imaged” to the PMD at the output through thesame transformation that governs birefringence. For a fiber consistingof two sections having respective PMDs τ₁ and τ₂, and respectiverotations of the SOP via finite rotations R₁ and R₂, the total PMD canbe represented by:τ=τ₂ +R ₂τ₁  (2)This equation states that the final PMD vector is represented by thevectorial sum of the second (i.e. final) section's PMD vector and thefirst section's PMD vector, but only after that first PMD vector hasbeen rotated by the same rotation operator (R₂) that rotates the SOPspropagating at that wavelength. This is shown by noting the rotations byβ implied in Eqns. 1a and 1c.

A generalization of Eqn. 2 shows that a similar rule applies for a fiberhaving multiple sections. Thus, each section of length Δz can beconsidered as having it's own uniform primitive PMD vector, β′Δz. ThePMD of the entire multi-sectioned fiber can be characterized as a vectorsum of the transformed primitive PMDs, one for each section, where eachPMD primitive vector is transformed by the concatenated rotation of allthe sections between it and the output. Since each of these constituentvectors is only a transformed version of its corresponding primitive PMDvector, each has the same length as its primitive vector, buteffectively suffers a random rotation (the Euler's theorem equivalent ofthe concatenated rotations between the section and the output). Thisprocess is illustrated in FIG. 1, where for an arbitrary opticalfrequency ω₀, the fiber (hereinafter, the optical fiber will be referredto as optical fiber) 100 is segregated into five independent sections(i.e., A, B, C, D, E), where each section's PMD is represented by avector (row 102) directly below that section, and these PMD vectorsrepresent a random distribution in magnitude and direction for therespective sections of the optical fiber. Each section's PMD vector(except the last one's) is imaged to the end and is shown on the rightside of the figure (at 106) as a primed version of the original. Thus,the PMD vector for section B is propagated through sections C, D, and E,resulting in its output image, vector B′. The PMD for the entire fiberis then computed as the vector sum of these constituents as depicted at108 in FIG. 1.

Referring now to FIG. 2, the PMD of the same fiber is shown at aslightly different optical frequency, ω₀+Δω. In this example, in row 202the PMD for each section at ω₀ (from FIG. 1) is represented by dottedvectors, while the PMD for each section at ω₀+Δω is represented by solidvectors. Each primitive vector corresponding to this neighboringfrequency (ω₀+Δω) is slightly different than the primitive vector forthe original frequency ω₀. This, by itself, results in a slightlydifferent sum for the total PMD vector at ω₀+Δω. However, in addition toslight changes in the primitive vectors, the new optical frequency alsocauses different rotations in each section, since the birefringence ineach section can also be a function of optical frequency. The images foreach section are imaged (trajectories 204) to the output at 206, and areslightly different from those depicted in FIG. 1 as shown by thedifference at 206 between the solid and dotted arrows. These change moredramatically as the optical frequency changes. In FIG. 2, the total PMDvector 208 at this new optical frequency is shown as a solid arrow,while the PMD vector at ω₀ (from FIG. 1) is depicted as a dotted arrow.Thus, the PMD will change in magnitude and direction as a function ofthe optical frequency, even though the constituent PMD vectors for thesections may be drawn from the same statistical ensemble representingthe fiber's properties. In large part, the study of PMD is a study ofthe properties of the statistics of the vector sum of these images.

Both the magnitude of the PMD vector, called the “differential groupdelay” or DGD, and the directions of the unit vectors parallel andanti-parallel to the PMD vector, called the “Principal States ofPolarization” (PSPs), change with optical frequency. The principalstates are orthogonal and thus are on opposite sides of the sphere. Theunit vector is usually associated with the slowest mode. Mostfrequently, it is the DGD which is plotted in discussions of PMD, butvariations in the PSPs with optical frequency also can cause distortionin the optical link. The properties of the PMD are therefore going tofollow the statistics of the sum of a set of vectors from the sectionsof the fiber that are chosen from a distribution and then, for the mostpart, randomly rotated after propagation through the fiber before beingsummed.

As discussed above, PMD fluctuates with changes in environmentalconditions. Even small environmental changes can add perturbations tothe birefringence of sections of the fiber and thereby move many of theimaged primitive vectors. This will consequently change the vector sum.It is to be expected that, at least for subtle environmental changes,the major effect is randomization of the individual rotations in each ofthe sections. However, since the original distribution was alreadyrandom, the statistical properties of the perturbed fiber are expectedto be essentially the same as those of the original fiber.

Based on the above, a “statistical ensemble” of the PMD can be studied.There are three common statistical ensembles from which observations canbe made of this random process: (1) an ensemble of identical, in thestatistical sense, fibers in which each primitive section of the fiberis drawn from a sample set, (2) the PMD of a specified fibertransmitting light at a particular wavelength over a very long period oftime (this implies the existence of environmental perturbations thatwill cause each section's primitive vector to change significantly overtime and thus also sample the distribution), and (3) the PMD of aspecific fiber at a given time over wavelength (where it is assumed thatthe wavelength spread is sufficient to cause each of the primitivevectors to change significantly over that wavelength spread). The“ergodic hypothesis” is that these three statistical ensembles will havethe same properties. This can be represented intuitively by a set ofimaged primitive vectors in the three cases. A multi-section fiber(representing the fiber of interest) can be constructed by drawingstatistical samples from either a distribution that represents thefiber) or, more generally, the distributions appropriate to the sectionif a length-dependent fiber is analyzed. Referring again to FIG. 1, astatistical ensemble of identical fibers will draw, for each section, anelement from the distribution, and that element will be imaged to theend of the fiber. That is, each section's representative will be imagedto the end and, as each element of the ensemble is examined, thatsection's contribution will be randomly rotated by the succeedingelements for each ensemble element. Similarly, over a long enough periodof time, the image from that section will also be rotated in a randomway because environmental perturbations over time will likewise causethe image to be rotated in a random manner. Thus, the average over anensemble and the average of an ensemble element over time will have thesame statistics.

For the case of PMD of a specific fiber at a given time averaged overwavelength, essentially the same distribution of primitive PMD vectorsexists. The rotations of the fiber sections vary with frequency becauseof the linear omega term in β (as discussed above, β=Δn(ω)ω/c) while thedirection and magnitude of the birefringence Δn is set by variations inthe slow and fast values of the refractive index, Δn=n_(s)−n_(f). Thus,the distributions at the wavelengths of interest will be substantiallythe same distributions as from the earlier samples. Although therotations corresponding to the various sections may change greatly dueto phase accumulation from the frequency changes, the net effect issimply another layer of randomization on an already random variable aslong as the rotations lead to further randomization. This furtherrandomization occurs if the change in optical frequency is large enough.Thus, the gross features of the statistical nature of PMD can becaptured by the statistics of a set of primitive PMD vectors for thefiber sections that are randomly imaged (i.e. rotated) to the end of thefiber and summed.

If, for example, the probability, P_(n) of a fiber system havingunacceptable PMD impairments, for a given channel, is 10⁻⁵ (the“natural” PMD outage probability) then from a statistical ensemble ofsuch fiber systems, 10⁻⁵ of them will experience outage at any giveninstant. In a statistical sense, this might mean that an outage of 5minutes [i.e., 1⁻⁵*(1 year)] can occur per year. Given an ensemble ofsystems that run for infinite duration, one would expect this to be truewhen calculating the expectation value for the outage. In real systems,however, it is much more likely that a channel will drift into an outagecondition and stay in that condition for some time. Thus, there will belong stretches of time in which the system is operational, perhaps somestretches in which the PMD varies rapidly (due to some perturbation)passing from one operational condition to another through an impairedcondition, but there will also be situations in which the system remainsimpaired for long periods of time. Averaged over very long periods oftimes, the outage may be 5 minutes per year, but in any given year, itis possible for the system to experience outages of less or much greaterduration.

Systems designers may be able to tolerate such impairments, for example,by deploying compensators that correct for the PMD impairments byintroducing the opposite PMD. This may lead to additional costs in WDMsystems, however, as the following example shows. If, for example, in aWDM system that has 100 wavelengths on a single fiber operating over alink with a natural PMD outage probability of 10⁻⁵, PMD that isexcessive enough to cause impairment is also likely to vary overwavelength and time, and thus place all channels in potential jeopardy.Accordingly, if one channel needs to be protected with a compensator,then all of them must compensated, or in time, they will all fail. Thesystem operator must therefore purchase 100 compensators, one for eachwavelength, even though the expected probability that even one of thecompensators might be used at any given time is roughly 0.001 (i.e.,100×10⁻⁵). The odds are therefore 1000 to one that none of thecompensators are needed at any given time, even though all 100 must bedeployed because each of them will eventually be needed at some time.

Co-owned Eiselt et al. (Eiselt) U.S. patent application Ser. No.09/729,954, filed Dec. 1, 2000, the disclosure of which is herebyincorporated herein, proposes a technique for mitigating the effects ofPMD that utilizes a plurality of polarization rotators in an opticalfiber to rotate the SOP of an optical signal. The Eiselt method is basedon the premise that if the system is about to fail, it is statisticallyin one of the “1000 to 1” cases (i.e., an unlikely “long-shot” conditionhas occurred). The Eiselt system changes the optical link slightly toeffectively “roll the dice again” in an attempt to hit one of thefavorable 999 cases. FIG. 3 conceptually depicts an optical medium 300segregated into a plurality of birefringent sections separated by thesubscripted polarization rotators “R” which are under system control.The polarization rotators are weak birefringent elements that can becontrolled to change the SOP of the signal in a prescribed manner. Thepolarization can be rotated as a result of the weak birefringence sothat, in effect, another element of the statistical ensemblecorresponding to that fiber can be chosen. In FIG. 3, the primitivevectors in row 302 for each of the sections are the same as those inFIG. 1. The polarization rotators rotate each section's primitivevectors to a new direction 304. As discussed above, each of these newprimitive vectors is imaged to the end of the fiber as represented inrow 306. These images are summed at 308 to represent the total PMDvector for the fiber with the polarization rotators. In this example,the total PMD vector is smaller than the original PMD (108) shown inFIG. 1 (and represented by the dashed line in FIG. 3), but it could havebeen larger. The rotators provide an extra level of randomization of theimage vectors, and thus will lead to a realization of PMD that isanother ensemble element of the original statistical ensemble, sinceeach section's contribution is drawn from the same distribution. When anew configuration is needed, the polarization rotators are set to newstates, thereby choosing another fiber realization from the ensemble.Thus, the polarization controllers essentially enable the choice ofanother ensemble member from the ensemble that represents the fiber.

The net effect is that by setting (possibly random) polarizationrotations in the fiber in this way, the current transmission line isreplaced with another one in the ensemble. If, as in the example above,the natural outage probability is 10⁻⁵ and since there are 100wavelengths to consider, the odds are overwhelming (e.g. 1000:1) thatthis new transmission line will not have a serious impairment. InEiselt, an error signal is generated when, by virtue of monitoring theerrors in all 100 channels, one of the channels is determined to besuffering a penalty, presumably due to PMD. This error signal can begenerated from SONET bytes that are running parity checks, or fromdiagnostics in the forward error correction (FEC) circuitry that is usedon many modern systems. By monitoring each of the FEC diagnosticchannels, the system can determine that one of its channels is nearing(or experiencing) an outage, it then resamples the ensemble space bysending commands to change the settings on the R rotators, hopefullylanding on one of the ensemble members for which all the channels areoperating.

SUMMARY OF THE INVENTION

In accordance with an aspect of the invention, there is provided amethod for mitigating PMD effects in optical transmission systems, whicheliminates the need for system feedback and threshold determination.

In accordance with another aspect of the invention, a method formitigating PMD effects in optical transmission systems is provided. Themethod generally comprises the steps of: receiving at least one opticalsignal from a first section of an optical fiber; continually rotatingthe polarization state of the at least one optical signal to randomlychange the polarization state of the at least one optical signal;propagating the at least one rotated optical signal through a secondsection of the optical fiber; receiving the at least one rotated opticalsignal; converting the at least one rotated optical signal into anelectrical signal; and correcting any bit errors which may have beenintroduced.

In accordance with another aspect of the invention, an opticalcommunications system that mitigates polarization mode dispersion (PMD)effects is provided. The system generally comprises: an optical fiberincluding a plurality of sections; an optical compensator disposedbetween adjacent sections of the optical fiber for continuously rotatingthe polarization state of at least one optical signal received from afirst of a pair of adjacent sections of the optical fiber signals torandomly change the polarization state of the at least one opticalsignal such that at least one rotated optical signal is communicated toa second of the pair of adjacent sections; an optical receiver forreceiving the at least one optical signal; and an error correctiondevice for correcting any bit errors which may have been introduced.

The present invention will now be described in detail with particularreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depicting PMD vectors representing a randomdistribution in magnitude and direction for the respective sections ofan optical fiber;

FIG. 2 is a schematic depicting the same fiber conducting an opticalsignal at a slightly different optical frequency, ω₀+Δω;

FIG. 3 is a schematic that conceptually depicts an optical mediumsegregated into a plurality of birefringent sections separated bypolarization rotators under system control;

FIG. 4 is a block diagram of an illustrative optical transmissionsystem;

FIG. 5 is a schematic of an exemplary long-haul WDM system; and

FIG. 6 is a schematic of an illustrative optical mitigation circuit inthe form of a Lefevre polarization controller.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference now to the several views of the drawings, there isdepicted a system for mitigating PMD in an optical transmission systemby continually scanning an ensemble of fibers.

In FIG. 4, there is depicted a block diagram of an illustrative opticaltransmission system 400, comprising an optical source 402, polarizationrotator 404 and an optical receiver 406. The polarization rotator (orpolarization scrambler) 404 receives optical signals from the opticalsource 402 via a first section 408 of an optical fiber, rotates thepolarization states of the optical signals, and then provides therotated optical signals to the optical receiver 406 via a second section410 of the optical fiber. The polarization rotator randomly rotates thepolarization states of the optical signals received via link 408 asdescribed in more detail hereinbelow. The optical source 402 and theoptical receiver 406 may be any one of a plurality of different types ofoptical sources and receiving devices, such as transmission systems withoptical transceivers or any known or later developed combination ofsoftware and hardware capable of generating and receiving, relayingand/or recalling from storage any information capable of beingtransmitted and received in an optical signal.

The optical receiver 406 may include hardware/software for measuring anerror condition such as the total number of bit-errors counted in areceived optical signal, and for correcting such errors by using, forexample, forward error correction (FEC) techniques. For example, biterrors can be measured and corrected in accordance with the ITU G.709standard, using a forward error correction (FEC) technique or accordingto any other known or later developed implementation capable ofcorrecting bit-errors in a received optical signal. In accordance withan aspect of the invention, a plurality of polarization rotators isdisposed between adjacent sections of an optical fiber and operatescontinuously to change the polarization state of the optical signal.This enables a continuous sample of the ensemble to be taken over time,thereby always reaching new fiber states that are represented bydifferent elements of the statistical ensemble. By utilizing manypolarization rotators that are spaced at approximately the correlationlength of the fiber (the length of a section over which thebirefringence is essentially uniform), the sampling will be close toideal in the sense that at any instant of time any one of the ensemblemembers is accessible. Thus, over a long enough period of time, theentire distribution is sampled. With reference again to FIG. 3, if theindividual segments represent sections of fiber that are less than orequal to a correlation length (i.e. the length over which the specificPMD is essentially constant), then locating polarization rotators ineach section will essentially sample all the degrees of freedom overwhich the PMD can be expressed and the sampling will be complete. If, onthe other hand, the sections in FIG. 3 are much longer than thecorrelation length, then the respective PMDs represented in each sectionare already partial sums as each section is already the concatenation ofmultiple independent subsections of fiber that are a correlation lengthlong, and independent rotations of those subsections (with respect toeach other) is not possible. The ensemble elements that are accessibleby changing the states of the polarization rotators are those in whichthe intermediate concatenations of subsections, those comprising thesections, are already fixed. Only those ensemble elements with the givenintermediate concatenations (but with independent joint sections) can beaccessed by changing the states of the polarization rotators.

Referring now to FIG. 5, there is depicted an exemplary long-haul WDMsystem 500 in which a plurality of optical signals having respectivewavelengths λ₁, λ₂ . . . λ_(n) are multiplexed via multiplexer 502 to anoptical fiber 504 that has been segregated into a plurality of spans,i.e., fiber sections L₁, L₂, L₃, . . . L_(N). The multiplexed signalsare demultiplexed at 505 as is well known in the art. The demultiplexer505 may include hardware/software for measuring an error condition suchas the total number of bit-errors counted in a received optical signal,and for correcting such errors by utilizing, for example, FEC. Aplurality of optical amplifiers 506 are disposed at locations definingthe terminating ends of each section L. Such amplifiers are generallyplaced to restore optical signal amplitudes before they have decayed toa level for which noise levels would corrupt the data. These amplifiersrequire power and are thus at locations in which other equipment(requiring electrical power) can be placed. A chromatic dispersioncompensation module 508 is operably coupled to each amplifier 506 tocompensate for the effects of chromatic dispersion in the fiber. In manysystems today, such compensators are placed mid-span in a multi-stageoptical amplifier. A polarization rotator 510 continuously rotates theoptical signal's polarization state. The polarization rotator 510 can bean electro-optic polarization controller that utilizes an electricaldrive signal of sufficiently high frequency. The polarization rotator510 may comprise one or more optical polarization controllers such as,for example, a number of fiber squeezers, a combination of λ/2 and λ/4optical delay components, or the like.

FIG. 6 depicts an illustrative polarization rotator 600 in the form of aLefevre polarization controller that receives a plurality of opticalsignals with incoming wavelengths and polarizations [λ₁(θ₁), λ₂(θ₂),λ₃(θ₃) . . . λ_(n)(θ_(n))] from a section 602 of the optical fiber,(where λ represents the wavelength of each signal and θ is meant torepresent its SOP, such as the polar and azimuthal angles in thePoincare sphere representation) and rotates the polarization state ofthe received signals to [λ₁(θ₁+ø₁), λ₂(θ₂+ø₂), λ₃θ₃+ø₃) . . .λ_(n)(θ_(n)+ø_(n))], where ø represents the change in the signal's SOP,such as might be represented in a Poincare sphere representation. Thepolarization-rotated signals are then communicated to an adjacentsection 604 of the optical fiber. Polarization rotation can beimplemented with a single polarization rotator that can rotate theoptical signal's SOP around a predetermined on a Poincare Sphere.Alternatively, a plurality of concatenated polarization rotators capableof rotating the polarization state around a plurality of fixed,orthogonal axes can be utilized. Euler's theorem states that thesedifferent physical views can both be viewed as a single rotation, so wewill take this viewpoint.

Returning to our exemplary system of FIG. 5, if a large number ofpolarization rotators 510 are provided along the optical fiber and it isassumed that a certain amount of elapsed time T_(s) is required toobtain a representative sample of the ensemble, over T_(s), both goodand bad ensemble elements (i.e. ensemble elements possessing low andhigh PMD) will be sampled in roughly the correct distribution, thedistribution of the entire ensemble. Because the sample isrepresentative, the expected number of errors in T_(s) will be roughlyequal to B*T_(s)*P_(n), where P_(n) is the “natural” outage probabilitythat would be expected for the system at bit rate B. That is, the biterror rate will be approximately P_(n), when observed over a time periodT_(s). Technically, each bit will see slightly different slates of thepolarization as it travels from system input to output. However,practically speaking, there is a certain correlation time window ofduration T_(c), in which the bits will see approximately the same levelof impairment. The errors in this window are essentially correlatedbecause the bits in this window propagate through a single ensemblemember. While it is conventional to refer to the state of the fiber at amoment in time as being determined by the polarization rotators, it isreally the state of the fiber as experienced in the retarded time of apropagating pulse that is of importance. Naturally, this T_(c)<<T_(s),since for the period T_(c), effectively one ensemble member is beingsampled, and it takes many samples to fully characterize thedistribution. Although performance may be degraded as errors occur atthe natural outage rate, P_(n), when averaged over a long enough time,forward error correction (FEC) techniques may be employed to correctthese errors and thus improve the overall system performance.

FEC enables the detection and correction of errors that occurinfrequently. A corrupted bit pattern can be restored to the patternthat was sent. As known by those skilled in the art, the basic conceptis that a series of data bits can be sent as a block, along withadditional coding bits. These augmented, coded, blocks have co-ordinatesin an abstract space. A bit from the block is detected by the detectoras “errored,” when the resulting vector representing the errored blockin the abstract space is not in the allowed set of vectors. Furthermore,the “distances,” in the abstract space between the resulting vector andthe allowed vectors are such that the intended vector can be determinedunambiguously as being the closest vector to the received, errored,vector. In addition, the coding can be concatenated, with interleavingbetween stages, so that even if there were consecutive errors (i.e. aburst of errors) they can be “striped” over multiple code words, each ofwhich would have only one error, for instance. Thus, interleavingincreases FEC's effectiveness to correcting bursts of consecutiveerrors.

As a result of PMD, an optical system can drift into a “bad” state inwhich severe impairments might occur for a long period of time. The PMDdwell time for typical optical links in field-installed systems can bemeasured in hours to weeks. The processing required to implement theinterleaving technique ultimately limits the length of an error burstthat can be corrected, as this is much shorter than the potential PMDdwell time. In accordance with an aspect of the invention as shown inFIG. 5 and described above, the polarization rotators continuouslyrotate the SOPs of the optical signals. If the polarization rotators aredriven at a fast enough rate, the system will dwell in a PMD erroredstate for a time shorter than the processing length of the FECcircuitry. That is, the number of bits that would potentially be erroreddue to PMD will be approximately BT_(c) and this would be within therange of the FEC processing Accordingly, FEC can correct errorsintroduced by PMD as long as it can correct error bursts ofapproximately BT_(c) bits in length.

The operation of this method is to be contrasted with the Eiseltapproach. They are similar in that they each use a multiplicity ofpolarization rotators in the links of a long-haul system, and that theycan beneficially use FEC. However, the Eiselt approach is essentiallystatic with respect to the polarization rotators (i.e. it tries to leavethe polarization rotators in a fixed condition) and uses the FECcircuitry's error rate reporting for each wavelength as an error signalin a feedback loop: when error rates on any one of the wavelengthsreaches a pre-determined threshold, the polarization rotators areenergized to provide a random set of rotations. The error rates areintended to stay at low levels for all wavelengths all the time. Thecurrent approach is complementary. It is active in the sense that thepolarization rotators are driven continuously, without regard to anyfeedback signals at all. This is simpler, in the sense that nointelligence or processing is needed, and no evaluation of the FEC errorrates is made to generate an error signal. On the other hand, instead ofdriving the system towards lower error rates, the present approachaccepts that random driving of the rotators will at some times actuallyincrease the system's uncorrected error rate. The parameters of therotation speeds, etc., must be chosen for a given system.

In the illustrative WDM system shown in FIG. 5, the rotation processillustrated in FIG. 3 and described above is implemented throughpolarization rotators at sites such as those illustrated in FIG. 5. Asin FIG. 2, the same fiber looks different to different wavelengths asthe light propagates. Similarly, as the polarization rotatorsillustrated schematically in FIGS. 3 and 5 are driven, the character ofthe fiber changes over time. Accordingly, the continuous rotation of thesignal's polarization at a specified wavelength as shown in FIG. 3occurs on all wavelengths as depicted in FIG. 5, although each signalwavelength will have consequently different vectors and birefringencerotations. Each wavelength channel may be experiencing high or low PMDat any given point in the bit stream, while other channels may beexperiencing markedly different impairment levels over the same retardedperiod of time. Over long periods of time, however, each channel willsample the PMD distribution, each channel will experience error burstsas the polarization rotators place the bits in that dwell time through ahighly impaired state, and each channel's FEC circuitry will correctthese errors at different times. At other times, each wavelength will bein a low PMD, low error state. In the aggregate, though, each channelwill have an approximate BER of P_(n) that will be corrected by the FECcircuitry.

There is a tradeoff between the driving rate for the polarizationcontrollers and the transmission system's clock recovery circuit that ismediated by the number of errored bits that the FEC circuit can process.As a rough estimate, we note that systems are typically specified towork at mean DGDs that are approximately ⅛ of a bit period. This meansthat the vast preponderance of time delay variations caused byvariations in the DGD is encompassed in about ½ of a bit period, sincethe DGD has a Maxwellian distribution. As the polarization rotators aredriven, all these variations in DGD will appear at the receiver to bevariations in the pulse arrival times. The instantaneous clock phasewill therefore vary as the polarization rotators sample the ensemble,imposing a different DGD (and thus arrival time) with each ensembleelement. Over time interval T_(c), the shortest time scalecharacterizing changes in PMD (and therefore DGD and arrival time) therewill nominally be BT_(c) bits, and this is also equal to or less thanthe FEC processing power. Over that same T_(c) time period there will beas much as another ½ bit variation, due to the ½ period variation inDGD, so that now T_(c), contains B T_(c)+/−½ bits. Thus, the fractionalchange in instantaneous frequency is approximately BT_(c), or, putanother way, if the FEC can correct for a burst of N bits, then BT_(c)≦N, and the clock phase changes by ½N. Thus, if the FEC circuitrycan correct for a burst of 1024 bits, for example, the clock phase willjitter by approximately 500 bit periods.

The present invention has been shown and described in what areconsidered to be the most practical and preferred embodiments. It isanticipated, however, that departures may be made therefrom and thatobvious modifications will be implemented by those skilled in the art.

1. A method of mitigating polarization mode dispersion (PMD) effects inan optical communications system, comprising the steps of: receiving atleast one optical signal from a first section of an optical fiber;continually rotating the polarization state of the at least one opticalsignal to randomly change the polarization state of the at least oneoptical signal; propagating the at least one rotated optical signalthrough a second section of the optical fiber; receiving the at leastone rotated optical signal; converting the at least one rotated opticalsignal into an electrical signal; and correcting any bit errors whichmay have been introduced.
 2. The method recited in claim 1, furthercomprising the steps of: receiving the at least one rotated opticalsignal from the second section of the optical fiber; continually furtherrotating the polarization state of the at least one optical signal torandomly change the polarization state of the at least one opticalsignal; and propagating the at least one further rotated optical signalthrough a third section of the optical fiber.
 3. The method recited inclaim 2, wherein the rotation of the at least one optical signalreceived from the first section is performed at a distance from anoptical source less than or equal to the correlation length of theoptical fiber, and the rotation of the at least one optical signalreceived from the second section of the optical fiber is performed at adistance from the rotation of the at least one optical signal receivedfrom the first section less than or equal to the correlation length ofthe optical fiber.
 4. The method recited in claim 1, wherein the step ofcorrecting bit errors utilizes forward error correction.
 5. A method ofmitigating polarization mode dispersion (PMD) in an opticalcommunications system, comprising the steps of: propagating at least oneoptical signal received from an optical source through a first sectionof an optical fiber; receiving the at least one optical signal from thefirst section of the optical fiber; continually rotating thepolarization state of the at least one optical signal at a distance froman optical source less than or equal to the correlation length of theoptical fiber; propagating the at least one rotated optical signalthrough a second section of the optical fiber over a distance less thanor equal to the correlation length; receiving the at least one rotatedoptical signal from the second section of the optical fiber; continuallyfurther rotating the polarization state of the at least one opticalsignal; and propagating the at least one further rotated optical signalthrough a third section of the optical fiber.
 6. The method recited inclaim 5, further comprising the steps of: receiving the further rotatedoptical signal from the third section of the fiber, and applying forwarderror correction to the at least one further rotated optical signal. 7.A method of compensating for polarization mode dispersion (PMD) effectsin an optical communications system, comprising the steps of:propagating a plurality of multiplexed optical signals received from anoptical source through a first section of an optical fiber; receivingthe plurality of optical signals from the first section of the opticalfiber; continually rotating the polarization states of the plurality ofoptical signals to randomly change the polarization states of theoptical signals at a distance from the optical source less than or equalto the correlation length of the optical fiber; propagating the rotatedplurality of optical signals through a second section of the opticalfiber over a distance less than or equal to the correlation length;receiving the rotated plurality of optical signals from the secondsection of the optical fiber; continually further rotating therespective polarization states of the plurality of optical signals torandomly change the polarization states of the optical signals;propagating the further rotated plurality of optical signals through athird section of the optical fiber; receiving the rotated opticalsignals; converting the rotated optical signals into electrical signals;and correcting any bit errors which may have been introduced.
 8. Amethod of mitigating polarization mode dispersion (PMD) effects in anoptical communications system, comprising the steps of: continuallyrotating the polarization states of a plurality of multiplexed opticalsignals to randomly change the polarization states of the opticalsignals between first and second adjacent sections of an optical fiber;propagating the rotated plurality of optical signals through the secondof the adjacent sections of the optical fiber; receiving the rotatedoptical signals; converting the rotated optical signals into electricalsignals; and correcting any bit errors which may have been introduced.9. An optical communications system that mitigates polarization modedispersion (PMD) effects, comprising: an optical fiber including aplurality of sections; an optical compensator disposed between adjacentsections of the optical fiber for continuously rotating the polarizationstate of at least one optical signal received from a first of a pair ofadjacent sections of the optical fiber signals to randomly change thepolarization state of the at least one optical signal such that at leastone rotated optical signal is communicated to a second of the pair ofadjacent sections; an optical receiver for receiving the at least oneoptical signal; and an error correction device for correcting any biterrors which may have been introduced.
 10. The optical communicationssystem recited in claim 9, wherein the sections of the optical fiberhave a length that is less than or equal to a correlation length. 11.The optical communications system recited in claim 9, wherein theoptical receiver applies forward error correction to the at least onereceived optical signal.
 12. The optical communications system recitedin claim 9, further comprising an optical source that transmits aplurality of multiplexed optical signals to the optical fiber.
 13. Anoptical communications system that mitigates polarization modedispersion (PMD) effects, comprising: an optical source that transmits aplurality of multiplexed optical signals; an optical fiber coupled tothe optical source, the optical fiber including a plurality of sections;an optical compensator disposed between adjacent sections of the opticalfiber for continuously rotating the polarization state of at least oneoptical signal received from a first of a pair of adjacent sections ofthe optical fiber to randomly change the polarization state of the atleast one optical signal such that at least one rotated optical signalis communicated to a second of the pair of adjacent sections; an opticalreceiver for receiving the at least one optical signal; and an errorcorrection device for correcting any bit errors which may have beenintroduced.